Aortic cannulas are used to return blood or provide other fluid to the aorta while the heart is by-passed during heart surgery. These cannulas are purposely made with small diameters (typically six to eight millimeters, but even smaller for pediatric applications) to minimize the disruption to the aorta, which in many heart surgery patients have advanced complex atherosclerotic lesions with adherent blood thrombi. The flow velocities through these small diameter cannula must be very high in order to maintain a satisfactory blood flow rate of about five to seven liters per minute. In at least some styles of conventional aortic cannula now in use, this high velocity resulted in "jet" flow emanating from the distal end of the cannula, which acted as a nozzle. It is believed that the force of this narrow jet stream may dislodge atheromatous material and/or adherent thrombi from the walls of the aorta, causing embolisms. As surgical equipment and techniques improve, making heart surgery available to older and more seriously ill patients, thromboatheroembolisms affect an increasing number of patients due to the increasing extent of atherosclerosis with age.
The size of aortic cannula may be constrained by the constricted size of the aorta of the typical heart surgery patient. Moreover, the ability to diffuse flow is restricted by the fragility of the blood, which is easily damaged by the shear stresses associated with turbulence.
The aortic cannulas of the present invention are adapted to provide high volume flow at relatively lower exit flow velocities than the conventional aortic cannulas presently available, thereby reducing the jet flow and consequently reducing the incidence of thromboatheroembolisms. Generally aortic cannulas constructed according to the principles of this invention comprise a plurality of spiral or helical slits in the distal tip to form a plurality of outlet openings in the sidewall of cannula adjacent the distal end. The slits open and close in response to pressure in the cannula, preventing a high pressure build up, and thereby reducing jet flow from the cannula. The slots allow the flow to quickly establish a stable, more uniform velocity flow.
Thus, the aortic cannula of the present invention reduces the high velocity jetting that can occur with some conventional aortic cannulas, while maintaining flow rate and minimizing damage to the blood.
These and other features and advantages will be in part apparent and in part pointed out hereinafter.